This invention relates to electrostatic separation and triboelectric charging.
The goal of durable goods plastics recycling is to separate the various types of plastic in a waste stream so that the plastics can be reused. This task is daunting because of the complexity of waste feed stocks, which typically have waste products as diverse as electronic goods, automotive parts, and consumer products, such as containers. The variety of plastics include polycarbonate (PC), polypropylene (PP), polystyrene (PS), acetylene-butadiene-styrene (ABS), high-impact polystyrene (HIPS), polyvinyl chloride (PVC), polyamide (PA), high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), polyurethane (PUR), polyesters, polyphenylene oxide (PPO) and polybutylene terephthalate (PBT) and others. In addition, the products can include various grades of these polymers and the blends of polymers such as PC/ABS, PVC/ABS and others.
One technique that has been suggested for separating plastic waste streams is electrostatic separation. In this technique, the plastic particles in the waste stream are charged and passed through high voltage electric fields where they are differentially deflected based on the polarity of the charge or the magnitude of the charge. One method for charging the materials is triboelectric charging. Triboelectric charging occurs when two dissimilar materials are allowed to rub together and electrons are transferred from one material to another. The direction of electron transfer and the magnitude of the relative voltage can be obtained by rubbing materials together and measuring the electrostatic voltages.
Triboelectrostatic separation has gained some successful applications in separating waste plastics from beverage bottles, PVC from PET, and packaging plastics, PE from PP. The separation efficiency can dramatically decrease when processing mixtures consisting of more than two materials, or when the relative composition of the two components is not constant.
This invention relates to improving the separation efficiency in a triboelectric separator by adding media against which the components of the mixture will charge. As a result, random charging between the components of the mixture is reduced and controlled, and predictable charging is achieved.
In general, the invention features a method of triboelectric treatment of a mixture that includes charging the mixture with a media. The media may contain a single or multiple components. It is selected in accordance with the electrostatic charging sequence of the components in the mixture, and the expected products from the mixture. The charging sequence is generally described in Chen C. Ku and Raimond Liepins, Electrical Properties of Polymers (1987).
Preferred embodiments may include one or more of the following features. The media component may be composed of a material, e.g., a polymer material, that is substantially the same as a component in the mixture. Alternatively, the media component may be substantially composed of material that is dissimilar from the components of the mixture. The terms xe2x80x9csamexe2x80x9d and xe2x80x9cdissimilarxe2x80x9d refer, for example, to chemical composition, grade, etc. The media component may be provided in an amount that is at least about 50% of the mixture by volume or mass, or in an amount that is less than 50% of the mixture by volume or mass. For example, the amount of media component may be at least about 3 times the mass or volume of the mixture. The particle size of the media may be selected such that it is the same as, or substantially different from, the particle size of the components of the mixture. The position on the charging sequence of the media may be the same as, or different from, the components of the mixture.
The media may be combined with the mixture to effect charging and then separated from the mixture after charging. Suitable separation processes include dry processes (e.g., air classification, screening, and combinations thereof) and wet processes (e.g., gravity concentration, froth floatation, and combinations thereof). Separation of the media from the mixture may occur either before or after the media has passed through an electric field. The separated media may be recycled, e.g., in a continuous loop.
The mixture may be a two component mixture or a multi-component mixture, e.g., having three or more components. The mixture may contain polymeric, mineral, agricultural or other components. An example of a suitable mixture is a durable products waste stream such as granulated electrical component waste stream.
Embodiments of the invention may also include one or more of the following advantages. The media can completely control the triboelectric charging of the mixture. Any component in the mixture could charge against the media, while charging between components of the mixture is negligible. Therefore, the variation in component percentages would not significantly affect the charging or the separation. In other words, any material, whatever its percentage in the composition, would charge against the media to the same extent. In this way, random charging is minimized, and improved separation is achieved by controlled charging. Highly complex polymer mixtures that do not include a primary component that dominates the mixture can be effectively separated. The technique would minimize the requirement to adjust splitter position in an electrostatic separator to accommodate changes in feed composition. The technique may enable a ternary separation that generates three products simultaneously in one pass, so as to reduce the number of separation steps required to produce multiple products. The technique is adaptable to waste streams in which the component concentrations in the mixture vary over time. The media is generally non-polluting and can be recycled in a continuous stream.
Further aspects, features and advantages follow.